A Multi-Input Multi-Output (MIMO) communication system uses a plurality of channels in a spatial area. Antenna arrays that have multiple transmission antennas can increase the capacity of data transmission through MIMO transmission schemes.
Two types of MIMO transmission schemes can be employed. Single-user MIMO (SU-MIMO) involves transmitting at least one data stream to a single user over a time frequency resource. Multi-user MIMO (MU-MIMO) involves transmitting at least one data stream per user to at least two co-scheduled users over a single, i.e., same time frequency, resource.
In a MIMO communication system, base stations and mobile stations use codebooks to enable channel state information feedback. The codebooks may additionally be used for precoding the information streams at the transmitter. Elements of a codebook are stored in both base stations and in mobile stations, and can be used to quantize the spatial channel state information for feedback. Each codebook element is a vector or a matrix, depending on the dimension of the channel matrix and the number of data streams supported. When communicating with a base station, the mobile station receives a Channel State Information Reference Signal (CSI-RS) from the base station and, using the CSI-RS, determines the state of the channel (generally referred to as Channel State Information (CSI)) between it and the base station and, based on the channel state, selects a vector or a matrix from the codebook. The mobile station then “recommends” the selected vector or matrix to the base station as part of overall CSI feedback. The base station may then use the recommended vector for beamforming or, more generally, the recommended matrix for precoding data streams prior to transmission via multiple antennas. Precoding is a technique that is used to weight multiple data streams transmitted from an antenna array in order to maximize the throughput of the link.
Typically, MIMO systems support a maximum of eight CSI-RS ports. However, some base station antenna arrays may employ more than eight antenna elements, which exceeds the number of CSI-RS antenna ports available. Furthermore, large antenna arrays may require additional CSI, including precoding vector or matrix recommendations, from connected mobile stations.